In conventional measurements of the neural activity of a living body, electrodes using such materials as biomolecular collagen and metallic silver wires are contacted to a nerve fascicle, and the measurement of a minimal current flow in the nerve fascicle is performed through insulated wires affixed to the electrode. These electrodes are used for the measurements of neural activities including peripheral nerves having the diameter of the order of millimeters, and sympathetic nerves to the heart and kidney having the diameter of the order of several hundreds micrometers. However, the sympathetic nerves to the other visceral organs such as the adrenal, splenic, and digestive organs etc. or the nerves distributing to skeletal muscles, skin, and brain/spinal chord have extremely smaller diameters on the order of less than 100 μm, and they are vulnerable to mechanical damages. Furthermore, it is difficult to maintain good insulation performance in the living body for longer than about one week, for which conventional measurements can be achieved. Finally, low signal levels of some microvolts measured in the small nerve bundle cause difficulties in achieving reliable signal processing.
As for an in-vivo electrode, which may enhance the reliability of signal processing, a microelectrode is disclosed by Japanese Unexamined Patent Publication No. 157669/2000. The microelectrode has characteristic as shown in FIGS. 10a˜c and a signal processing electrical circuit assembled in the vicinity of the microelectrode. The patent application discloses a crank like electrode 101 having a horseshoe shape into which a neural axon bundle 100 is led in between with contiguity. Further in the application, a furcated electrode (See FIG. 10b-102) and a needle like electrode (See FIG. 10c-103) stabbing the neural bundle 100 are disclosed respectively.
Any of the crank like, furcated, and needle like microelectrodes described above can be applied only to a thick nerve bundle and has limits in stable measurements of neural activity. Further, the measuring object is a microscopic-level nerve fascicle in the living body; which can easily be mechanically damaged or killed. Furthermore, it is difficult to maintain good insulation performance of an implantable electrode for a long period of time and to keep it stably fitted to the thin nerve fascicle. Especially for the purpose of elucidating the neural regulation of the cardiovascular system, which is controlled by the brain and autonomic nerve system, it is necessary to simultaneously measure multiple autonomic nerve activities, which governs the above-described visceral organs. This invention is directed to provide an ultra-miniature in-vivo electrode, which facilitates multidimensional measurements without any damage to a thin nerve bundle. Moreover, this invention is directed to provide an ultra-miniature in-vivo electrode, which has high insulation performance facilitating long-term stable measurements.